ufs: kill lock_ufs()

There were 3 remaining users; in two of them we took ->s_lock immediately
after lock_ufs() and held it until just before unlock_ufs(); the third
one (statfs) could not be called from itself or from other two (remount
and sync_fs).  Just use ->s_lock in statfs and don't bother with lock_ufs
at all.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>