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.