isofs: avoid unused function warning

With the isofs_hash() function removed, isofs_hash_ms() is the only user
of isofs_hash_common(), but it's defined inside of an #ifdef, which triggers
this gcc warning in ARM axm55xx_defconfig starting with v3.18-rc3:

fs/isofs/inode.c:177:1: warning: 'isofs_hash_common' defined but not used [-Wunused-function]

This patch moves the function inside of the same #ifdef section to avoid that
warning, which seems the best compromise of a relatively harmless patch for
a late -rc.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: b0afd8e5 ("isofs: don't bother with ->d_op for normal case")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>