fuse2fs: fix in_file_group missing the primary process gid

fuse2fs: fix in_file_group missing the primary process gid

I forgot that Unix processes have both a primary group id and a list of
supplementary group ids.  The primary is provided by the fuse client;
the supplemental groups are noted by the Groups: field of
/proc/self/status.

If a process does not have /any/ supplemental group ids, then
in_file_group returns the wrong answer if the inode gid matches the
group id provided by the fuse client because it doesn't check that
anymore.  Make it so the primary group id check always happens.

Found by generic/375.

Cc: <linux-ext4@vger.kernel.org> # v1.47.3
Fixes: 3469e6ff606af8 ("fuse2fs: fix group membership checking in op_chmod")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>