tmpfs: make lseek(SEEK_DATA/SEK_HOLE) return ENXIO with a negative offset

Other filesystems such as ext4, f2fs and ubifs all return ENXIO when
lseek (SEEK_DATA or SEEK_HOLE) requests a negative offset.

man 2 lseek says

:      EINVAL whence  is  not  valid.   Or: the resulting file offset would be
:             negative, or beyond the end of a seekable device.
:
:      ENXIO  whence is SEEK_DATA or SEEK_HOLE, and the file offset is  beyond
:             the end of the file.

Make tmpfs return ENXIO under these circumstances as well.  After this,
tmpfs also passes xfstests's generic/448.

[[email protected]: rewrite changelog]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Yufen Yu <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: William Kucharski <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Yufen Yu

mm/shmem.c

@@ -2563,9 +2563,7 @@ static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
 	inode_lock(inode);
 	/* We're holding i_mutex so we can access i_size directly */
 
-	if (offset < 0)
-		offset = -EINVAL;
-	else if (offset >= inode->i_size)
+	if (offset < 0 || offset >= inode->i_size)
 		offset = -ENXIO;
 	else {
 		start = offset >> PAGE_SHIFT;