XFS: Free buffer pages array unconditionally

The code in xfs_free_buf() only attempts to free the b_pages array if the
buffer is a page cache backed or page allocated buffer. The extra log buffer
that is used when the log wraps uses pages that are allocated to a different
log buffer, but it still has a b_pages array allocated when those pages
are associated to with the extra buffer in xfs_buf_associate_memory.

Hence we need to always attempt to free the b_pages array when tearing
down a buffer, not just on buffers that are explicitly marked as page bearing
buffers. This fixes a leak detected by the kernel memory leak code.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Alex Elder <aelder@sgi.com>