fuse2fs: fix gid inheritance on sgid parent directories

fuse2fs: fix gid inheritance on sgid parent directories

When a child file is created inside a setgid parent directory, the child
is supposed to inherit the gid of the parent, not the fsgid of the
creating process.  Fix this error, which was discovered by generic/633,
generic/696, and generic/697.

Cc: linux-ext4@vger.kernel.org # v1.43
Fixes: 81cbf1ef4f5dab ("misc: add fuse2fs, a FUSE server for e2fsprogs")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/175182663041.1984706.582232688757289460.stgit@frogsfrogsfrogs
Signed-off-by: Theodore Ts'o <tytso@mit.edu>