xfs: merge _xfs_buf_get_pages()

Only called from one place now, so merge it into
xfs_buf_alloc_pages(). Because page array allocation is dependent on
bp->b_pages being null, always ensure that when the pages array is
freed we always set bp->b_pages to null.

Also convert the page array to use kmalloc() rather than
kmem_alloc() so we can use the gfp flags we've already calculated
for the allocation context instead of hard coding KM_NOFS semantics.

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