vfs: export lseek_execute() to modules

For those file systems(btrfs/ext4/ocfs2/tmpfs) that support
SEEK_DATA/SEEK_HOLE functions, we end up handling the similar
matter in lseek_execute() to update the current file offset
to the desired offset if it is valid, ceph also does the
simliar things at ceph_llseek().

To reduce the duplications, this patch make lseek_execute()
public accessible so that we can call it directly from the
underlying file systems.

Thanks Dave Chinner for this suggestion.

[AV: call it vfs_setpos(), don't bring the removed 'inode' argument back]

v2->v1:
- Add kernel-doc comments for lseek_execute()
- Call lseek_execute() in ceph->llseek()

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Chris Mason <chris.mason@fusionio.com>
Cc: Josef Bacik <jbacik@fusionio.com>
Cc: Ben Myers <bpm@sgi.com>
Cc: Ted Tso <tytso@mit.edu>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Sage Weil <sage@inktank.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/xfs/xfs_file.c

@@ -1268,8 +1268,7 @@ xfs_seek_data(
 	}
 
 out:
-	if (offset != file->f_pos)
-		file->f_pos = offset;
+	offset = vfs_setpos(file, offset, inode->i_sb->s_maxbytes);
 
 out_unlock:
 	xfs_iunlock_map_shared(ip, lock);
@@ -1377,8 +1376,7 @@ out:
 	 * situation in particular.
 	 */
 	offset = min_t(loff_t, offset, isize);
-	if (offset != file->f_pos)
-		file->f_pos = offset;
+	offset = vfs_setpos(file, offset, inode->i_sb->s_maxbytes);
 
 out_unlock:
 	xfs_iunlock_map_shared(ip, lock);