xfs: calculate inode walk prefetch more carefully

The existing inode walk prefetch is based on the old bulkstat code,
which simply allocated 4 pages worth of memory and prefetched that many
inobt records, regardless of however many inodes the caller requested.
65536 inodes is a lot to prefetch (~32M on x64, ~512M on arm64) so let's
scale things down a little more intelligently based on the number of
inodes requested, etc.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>