cifs: Fix check for regular file in couldbe_mf_symlink()

MF Symlinks are regular files containing content in a specified format.

The function couldbe_mf_symlink() checks the mode for a set S_IFREG bit
as a test to confirm that it is a regular file. This bit is also set for
other filetypes and simply checking for this bit being set may return
false positives.

We ensure that we are actually checking for a regular file by using the
S_ISREG macro to test instead.

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Reported-by: Neil Brown <neilb@suse.de>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <smfrench@gmail.com>