cifs: Fix removexattr for os2.* xattrs

If cifs_removexattr finds a "user." or "os2." xattr name prefix, it
skips 5 bytes, one byte too many for "os2.".

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>