Don't restore old mount options and flags if VFS_MOUNT(9) succeeds but
vfs_export() fails. Restoring old options and flags after successful
VFS_MOUNT(9) call may cause the file system internal state to become
inconsistent with mount options and flags. Specifically the FFS super
block fs_ronly field and the MNT_RDONLY flag may get out of sync.

PR: kern/133614
Discussed on: freebsd-hackers