xfs: merge xfs_buf_find() and xfs_buf_get_map()

xfs: merge xfs_buf_find() and xfs_buf_get_map()

Now that we factored xfs_buf_find(), we can start separating into
distinct fast and slow paths from xfs_buf_get_map(). We start by
moving the lookup map and perag setup to _get_map(), and then move
all the specifics of the fast path lookup into xfs_buf_lookup()
and call it directly from _get_map(). We the move all the slow path
code to xfs_buf_find_insert(), which is now also called directly
from _get_map(). As such, xfs_buf_find() now goes away.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>