fs/locks.c: Remove stale FIXME left over from BKL conversion

The comment is no longer true as (now that the BKL conversion is
finished) a spinlock _is_ now used to protect file_lock_list,
blocked_list and inode->i_flock.

Signed-off-by: Matt Fleming <matt.fleming@linux.intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>