libxfs: don't use enum for buffer flags

libxfs: don't use enum for buffer flags

This roughly mirrors

  807cbbdb xfs: do not use emums for flags used in tracing

and lets us use the xfs_buf_flags_t type in function calls as is
done in the kernel.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>