memory leak in nfs_fs_context_parse_monolithic

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    0bf999f9 linux/pipe_fs_i.h: fix kernel-doc warnings after ..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1589bd4ee00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2802e33434f4f863
dashboard link: https://syzkaller.appspot.com/bug?extid=193c375dcddb4f345091
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13dec65ee00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14c05311e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+193c375dcddb4f345091@xxxxxxxxxxxxxxxxxxxxxxxxx

executing program
BUG: memory leak
unreferenced object 0xffff8881166b3ac0 (size 32):
  comm "syz-executor191", pid 7127, jiffies 4294946216 (age 12.850s)
  hex dump (first 32 bytes):
    00 66 73 00 00 00 00 00 00 00 00 00 00 00 00 00  .fs.............
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<000000006202a794>] kmemleak_alloc_recursive include/linux/kmemleak.h:43 [inline]
    [<000000006202a794>] slab_post_alloc_hook mm/slab.h:586 [inline]
    [<000000006202a794>] slab_alloc mm/slab.c:3320 [inline]
    [<000000006202a794>] __do_kmalloc mm/slab.c:3654 [inline]
    [<000000006202a794>] __kmalloc_track_caller+0x165/0x300 mm/slab.c:3671
    [<000000007e324303>] kstrdup+0x3a/0x70 mm/util.c:60
    [<000000002835aac3>] nfs23_parse_monolithic fs/nfs/fs_context.c:958 [inline]
    [<000000002835aac3>] nfs_fs_context_parse_monolithic+0x5cf/0x8f0 fs/nfs/fs_context.c:1147
    [<000000004fe3f934>] parse_monolithic_mount_data+0x37/0x40 fs/fs_context.c:679
    [<00000000dccb779f>] do_new_mount fs/namespace.c:2818 [inline]
    [<00000000dccb779f>] do_mount+0x945/0xc80 fs/namespace.c:3107
    [<000000006234487e>] __do_sys_mount fs/namespace.c:3316 [inline]
    [<000000006234487e>] __se_sys_mount fs/namespace.c:3293 [inline]
    [<000000006234487e>] __x64_sys_mount+0xc0/0x140 fs/namespace.c:3293
    [<00000000d2937fdd>] do_syscall_64+0x73/0x220 arch/x86/entry/common.c:294
    [<00000000fa6ba8f3>] entry_SYSCALL_64_after_hwframe+0x44/0xa9



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