dm raid: don't use 'const' in function return

A newly introduced function has 'const int' as the return type,
but as "make W=1" reports, that has no meaning:

drivers/md/dm-raid.c:510:18: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]

This changes the return type to plain 'int'.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 33e53f06 ("dm raid: introduce extended superblock and new raid types to support takeover/reshaping")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>