xfs: remove the kmalloc to page allocator fallback

xfs: remove the kmalloc to page allocator fallback

Since commit 59bb47985c1d ("mm, sl[aou]b: guarantee natural alignment
for kmalloc(power-of-two)", kmalloc and friends guarantee that power of
two sized allocations are naturally aligned.  Limit our use of kmalloc
for buffers to these power of two sizes and remove the fallback to
the page allocator for this case, but keep a check in addition to
trusting the slab allocator to get the alignment right.

Also refactor the kmalloc path to reuse various calculations for the
size and gfp flags.

Signed-off-by: Christoph Hellwig <hch@lst.de>