ubifs: Fix memory leak in alloc_wbufs()

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

ubifs: Fix memory leak in alloc_wbufs()

kmemleak reported a sequence of memory leaks, and show them as following:

unreferenced object 0xffff8881575f8400 (size 1024):
comm "mount", pid 19625, jiffies 4297119604 (age 20.383s)
hex dump (first 32 bytes):
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
backtrace:
[<ffffffff8176cecd>] __kmalloc+0x4d/0x150
[<ffffffffa0406b2b>] ubifs_mount+0x307b/0x7170 [ubifs]
[<ffffffff819fa8fd>] legacy_get_tree+0xed/0x1d0
[<ffffffff81936f2d>] vfs_get_tree+0x7d/0x230
[<ffffffff819b2bd4>] path_mount+0xdd4/0x17b0
[<ffffffff819b37aa>] __x64_sys_mount+0x1fa/0x270
[<ffffffff83c14295>] do_syscall_64+0x35/0x80
[<ffffffff83e0006a>] entry_SYSCALL_64_after_hwframe+0x46/0xb0

unreferenced object 0xffff8881798a6e00 (size 512):
comm "mount", pid 19677, jiffies 4297121912 (age 37.816s)
hex dump (first 32 bytes):
6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
backtrace:
[<ffffffff8176cecd>] __kmalloc+0x4d/0x150
[<ffffffffa0418342>] ubifs_wbuf_init+0x52/0x480 [ubifs]
[<ffffffffa0406ca5>] ubifs_mount+0x31f5/0x7170 [ubifs]
[<ffffffff819fa8fd>] legacy_get_tree+0xed/0x1d0
[<ffffffff81936f2d>] vfs_get_tree+0x7d/0x230
[<ffffffff819b2bd4>] path_mount+0xdd4/0x17b0
[<ffffffff819b37aa>] __x64_sys_mount+0x1fa/0x270
[<ffffffff83c14295>] do_syscall_64+0x35/0x80
[<ffffffff83e0006a>] entry_SYSCALL_64_after_hwframe+0x46/0xb0

The problem is that the ubifs_wbuf_init() returns an error in the
loop which in the alloc_wbufs(), then the wbuf->buf and wbuf->inodes
that were successfully alloced before are not freed.

Fix it by adding error hanging path in alloc_wbufs() which frees
the memory alloced before when ubifs_wbuf_init() returns an error.