tmpfs: fix SEEK_DATA/SEEK_HOLE regression

Commit 46a1c2c7 ("vfs: export lseek_execute() to modules") broke the
tmpfs SEEK_DATA/SEEK_HOLE implementation, because vfs_setpos() converts
the carefully prepared -ENXIO to -EINVAL.  Other filesystems avoid it in
error cases: do the same in tmpfs.
Signed-off-by: NHugh Dickins <hughd@google.com>
Cc: Jie Liu <jeff.liu@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -1798,7 +1798,8 @@ static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
 		}
 	}
 
-	offset = vfs_setpos(file, offset, MAX_LFS_FILESIZE);
+	if (offset >= 0)
+		offset = vfs_setpos(file, offset, MAX_LFS_FILESIZE);
 	mutex_unlock(&inode->i_mutex);
 	return offset;
 }