readahead: code cleanup

readahead: code cleanup

Rename file_ra_state.prev_page to prev_index and file_ra_state.offset to
prev_offset.  Also update of prev_index in do_generic_mapping_read() is now
moved close to the update of prev_offset.

[[email protected]: fix it]
Signed-off-by: Jan Kara <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: WU Fengguang <[email protected]>
Signed-off-by: Fengguang Wu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>