Patch from Hugh Dickins

shmem_getpage does need to lock its page (to secure it against
shmem_writepage), but it's easier for its callers if it unlocks
before returning.  The only caller who appeared to be using the
page lock was shmem_file_write, but it wasn't actually protecting
against anything - i_sem prevents concurrent writes and truncates,
and do_shmem_file_read was dropping the lock before copying anyway.