libxfs: fix atomic64_t detection on x86 32-bit architectures

libxfs: fix atomic64_t detection on x86 32-bit architectures

xfsprogs during compilation tries to detect if liburcu supports atomic
64-bit ops on the platform it is being compiled on, and if not it falls
back to using pthread mutex locks.

The detection logic for that fallback relies on _uatomic_link_error()
which is a link-time trick used by liburcu that will cause compilation
errors on archs that lack the required support. That only works for the
generic liburcu code though, and it is not implemented for the
x86-specific code.

In practice this means that when xfsprogs is compiled on 32-bit x86
archs will successfully link to liburcu for atomic ops, but liburcu does
not support atomic64_t on those archs. It indicates this during runtime
by generating an illegal instruction that aborts execution, and thus
causes various xfsprogs utils to be segfaulting.

Fix this by requiring that unsigned longs are at least 64 bits in size,
which /usually/ means that 64-bit atomic counters are supported.  We
can't simply execute the liburcu atomic64_t detection code during
configure instead of only relying on the linker error because that
doesn't work for cross-compiled packages.

Fixes: 7448af588a2e ("libxfs: fix atomic64_t poorly for 32-bit architectures")
Reported-by: Anthony Iliopoulos <ailiop@suse.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>