fuse: name fs_context consistently

[ Upstream commit 84c21507 ]

Naming convention under fs/fuse/:

	struct fuse_conn *fc;
	struct fs_context *fsc;

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Stable-dep-of: 525bd65a ("fuse: verify {g,u}id mount options correctly")
Signed-off-by: Sasha Levin <sashal@kernel.org>