btrfs: fix memory leak in btrfs_do_encoded_write()

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix memory leak in btrfs_do_encoded_write()

Local fuzzing of 6.12.94 has found the following memory leak:

Unreferenced object 0xffff888018050a80 (size 64):
comm "syz.0.17", pid 10297, jiffies 4294953601
hex dump (first 32 bytes):
00 10 00 00 00 00 00 00 01 00 00 00 00 00 00 00 ................
10 0a 05 18 80 88 ff ff 10 0a 05 18 80 88 ff ff ................
backtrace (crc a8a6fc29):
kmemleak_alloc_recursive include/linux/kmemleak.h:42 [inline]
slab_post_alloc_hook mm/slub.c:4152 [inline]
slab_alloc_node mm/slub.c:4197 [inline]
__kmalloc_cache_noprof+0x168/0x2c0 mm/slub.c:4358
kmalloc_noprof include/linux/slab.h:878 [inline]
extent_changeset_alloc fs/btrfs/extent_io.h:207 [inline]
qgroup_reserve_data+0x1c5/0x7d0 fs/btrfs/qgroup.c:4305
btrfs_qgroup_reserve_data+0x2e/0xb0 fs/btrfs/qgroup.c:4355
btrfs_do_encoded_write+0x92e/0x1040 fs/btrfs/inode.c:9746
btrfs_encoded_write fs/btrfs/file.c:1482 [inline]
btrfs_do_write_iter+0x280/0x610 fs/btrfs/file.c:1507
btrfs_ioctl_encoded_write+0x3d6/0x490 fs/btrfs/ioctl.c:4738
btrfs_ioctl+0x6f9/0xc90 fs/btrfs/ioctl.c:-1
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:906 [inline]
__se_sys_ioctl+0xf9/0x170 fs/ioctl.c:892
do_syscall_x64 arch/x86/entry/common.c:47 [inline]
do_syscall_64+0xbe/0x1a0 arch/x86/entry/common.c:78
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Unreferenced object 0xffff888018050a00 (size 64):
comm "syz.0.17", pid 10297, jiffies 4294953601
hex dump (first 32 bytes):
00 00 00 00 00 00 00 00 ff 0f 00 00 00 00 00 00 ................
90 0a 05 18 80 88 ff ff 90 0a 05 18 80 88 ff ff ................
backtrace (crc cb5c9580):
kmemleak_alloc_recursive include/linux/kmemleak.h:42 [inline]
slab_post_alloc_hook mm/slub.c:4152 [inline]
slab_alloc_node mm/slub.c:4197 [inline]
__kmalloc_cache_noprof+0x168/0x2c0 mm/slub.c:4358
kmalloc_noprof include/linux/slab.h:878 [inline]
kzalloc_noprof include/linux/slab.h:1014 [inline]
ulist_prealloc+0x9c/0x110 fs/btrfs/ulist.c:114
extent_changeset_prealloc fs/btrfs/extent_io.h:217 [inline]
__set_extent_bit+0x16b/0x1a70 fs/btrfs/extent-io-tree.c:1086
set_record_extent_bits+0x50/0x90 fs/btrfs/extent-io-tree.c:1821
qgroup_reserve_data+0x274/0x7d0 fs/btrfs/qgroup.c:4312
btrfs_qgroup_reserve_data+0x2e/0xb0 fs/btrfs/qgroup.c:4355
btrfs_do_encoded_write+0x92e/0x1040 fs/btrfs/inode.c:9746
btrfs_encoded_write fs/btrfs/file.c:1482 [inline]
btrfs_do_write_iter+0x280/0x610 fs/btrfs/file.c:1507
btrfs_ioctl_encoded_write+0x3d6/0x490 fs/btrfs/ioctl.c:4738
btrfs_ioctl+0x6f9/0xc90 fs/btrfs/ioctl.c:-1
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:906 [inline]
__se_sys_ioctl+0xf9/0x170 fs/ioctl.c:892
do_syscall_x64 arch/x86/entry/common.c:47 [inline]
do_syscall_64+0xbe/0x1a0 arch/x86/entry/common.c:78
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fix this by freeing an extent changeset before returning from
btrfs_do_encoded_write().