ext4slower: fix getting kallsyms address
ext4slower: fix getting kallsyms address

$ grep ext4_file_operations /proc/kallsyms
ffffffffc0331340 r ext4_file_operations	[ext4]

shodoco/bcc,romain-intel/bcc,iovisor/bcc,iovisor/bcc,mkacik/bcc,brendangregg/bcc,shodoco/bcc,shodoco/bcc,iovisor/bcc,iovisor/bcc,mkacik/bcc,mcaleavya/bcc,mkacik/bcc,shodoco/bcc,brendangregg/bcc,mkacik/bcc,brendangregg/bcc,iovisor/bcc,tuxology/bcc,romain-intel/bcc,mcaleavya/bcc,mcaleavya/bcc,shodoco/bcc,romain-intel/bcc,mcaleavya/bcc,tuxology/bcc,tuxology/bcc,romain-intel/bcc,mkacik/bcc,romain-intel/bcc,mcaleavya/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc