xfs: clean up inode state flag tests in xfs_blockgc_igrab

Clean up the definition of which inode states are not eligible for
speculative preallocation garbage collecting by creating a private
#define.  The deferred inactivation patchset will add two new entries to
the set of flags-to-ignore, so we want the definition not to end up a
cluttered mess.

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