xfs: fix memcpy fortify errors in BUI log format copying

Starting in 6.1, CONFIG_FORTIFY_SOURCE checks the length parameter of
memcpy.  Unfortunately, it doesn't handle flex arrays correctly:

------------[ cut here ]------------
memcpy: detected field-spanning write (size 48) of single field "dst_bui_fmt" at fs/xfs/xfs_bmap_item.c:628 (size 16)

Fix this by refactoring the xfs_bui_copy_format function to handle the
copying of the head and the flex array members separately.  While we're
at it, fix a minor validation deficiency in the recovery function.

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