xfs: simplify xfs_map_at_offset

Move the buffer locking into the callers as they need to do it
wether they call xfs_map_at_offset or not.  Remove the b_bdev
assignment, which is already done by get_blocks.  Remove the
duplicate extent type asserts in xfs_convert_page just before
calling xfs_map_at_offset.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>