vfs: export lseek_execute() to modules

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 <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: Josef Bacik <[email protected]>
Cc: Ben Myers <[email protected]>
Cc: Ted Tso <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Sage Weil <[email protected]>
Signed-off-by: Al Viro <[email protected]>