fs/cifs/: fix misspellings using codespell tool

A typo is found out by codespell tool in 251th lines of cifs_swn.c:

$ codespell ./fs/cifs/
./cifs_swn.c:251: funciton  ==> function

Fix a typo found by codespell.

Signed-off-by: Liu xuzhi <liu.xuzhi@zte.com.cn>
Signed-off-by: Steve French <stfrench@microsoft.com>