ufs: kill more lock_ufs() calls

a) move it inside ufs_truncate()
b) ufs_free_inode() doesn't need it - it's serialized on ->s_lock
c) ufs_write_inode() doesn't need it either (and can be called without
it anyway).

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