ncpfs: fix wrong check in __ncp_ioctl()

We want to check for s_inode's existence, not inode's one (inode is always
valid in this function).

This takes care of the following entry from Dan's list:

fs/ncpfs/ioctl.c +445 __ncp_ioctl(180) warning: variable derefenced before check 'inode'

Reported-by: Dan Carpenter <error27@gmail.com>
Cc: Julia Lawall <julia@diku.dk>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Cc: Petr Vandrovec <vandrove@vc.cvut.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>