vfs: remove duplicate code in get_fs_type()

save 14 bytes:

   text    data     bss     dec     hex filename
   1354      32       4    1390     56e fs/filesystems.o.before
   text    data     bss     dec     hex filename
   1340      32       4    1376     560 fs/filesystems.o

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>