btrfs: fix leak of kobject name for sub-group space_info

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

btrfs: fix leak of kobject name for sub-group space_info

When create_space_info_sub_group() allocates elements of
space_info->sub_group[], kobject_init_and_add() is called for each
element via btrfs_sysfs_add_space_info_type(). However, when
check_removing_space_info() frees these elements, it does not call
btrfs_sysfs_remove_space_info() on them. As a result, kobject_put() is
not called and the associated kobj->name objects are leaked.

This memory leak is reproduced by running the blktests test case
zbd/009 on kernels built with CONFIG_DEBUG_KMEMLEAK. The kmemleak
feature reports the following error:

unreferenced object 0xffff888112877d40 (size 16):
comm "mount", pid 1244, jiffies 4294996972
hex dump (first 16 bytes):
64 61 74 61 2d 72 65 6c 6f 63 00 c4 c6 a7 cb 7f data-reloc......
backtrace (crc 53ffde4d):
__kmalloc_node_track_caller_noprof+0x619/0x870
kstrdup+0x42/0xc0
kobject_set_name_vargs+0x44/0x110
kobject_init_and_add+0xcf/0x150
btrfs_sysfs_add_space_info_type+0xfc/0x210 [btrfs]
create_space_info_sub_group.constprop.0+0xfb/0x1b0 [btrfs]
create_space_info+0x211/0x320 [btrfs]
btrfs_init_space_info+0x15a/0x1b0 [btrfs]
open_ctree+0x33c7/0x4a50 [btrfs]
btrfs_get_tree.cold+0x9f/0x1ee [btrfs]
vfs_get_tree+0x87/0x2f0
vfs_cmd_create+0xbd/0x280
__do_sys_fsconfig+0x3df/0x990
do_syscall_64+0x136/0x1540
entry_SYSCALL_64_after_hwframe+0x76/0x7e

To avoid the leak, call btrfs_sysfs_remove_space_info() instead of
kfree() for the elements.