xfs: use static_assert to check struct sizes and offsets

Use the compiler-provided static_assert built-in from C11 instead of
the kernel-specific BUILD_BUG_ON_MSG for the structure size and offset
checks in xfs_ondisk.  This not only gives slightly nicer error messages
in case things go south, but can also be trivially used as-is in
userspace.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>