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With CONFIG_NFS_V4 and data version 4, nfs_get_sb will allocate memory for
export_path in nfs4_validate_text_mount_data, so we need to free it then.
This is addressed in following kmemleak report:

unreferenced object 0xffff88016bf48a50 (size 16):
  comm "mount.nfs", pid 22567, jiffies 4651574704 (age 175471.200s)
  hex dump (first 16 bytes):
    2f 6f 70 74 2f 77 6f 72 6b 00 6b 6b 6b 6b 6b a5  /opt/work.kkkkk.
  backtrace:
    [<ffffffff814b34f9>] kmemleak_alloc+0x60/0xa7
    [<ffffffff81102c76>] kmemleak_alloc_recursive.clone.5+0x1b/0x1d
    [<ffffffff811046b3>] __kmalloc_track_caller+0x18f/0x1b7
    [<ffffffff810e1b08>] kstrndup+0x37/0x54
    [<ffffffffa0336971>] nfs_parse_devname+0x152/0x204 [nfs]
    [<ffffffffa0336af3>] nfs4_validate_text_mount_data+0xd0/0xdc [nfs]
    [<ffffffffa0338deb>] nfs_get_sb+0x325/0x736 [nfs]
    [<ffffffff81113671>] vfs_kern_mount+0xbd/0x17c
    [<ffffffff81113798>] do_kern_mount+0x4d/0xed
    [<ffffffff81129a87>] do_mo...