[GFS2] fix file_system_type leak on gfs2meta mount

get_gfs2_sb does a get_fs_type without doing a put_filesystem and
thus leaking a file_system_type reference everytime it's called.

Just use gfs2_fs_type directly instead of doing the lookup and thus
fix the problem.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>