coda: ftoc validity check integration

This patch moves cfi check in coda_ftoc() instead of repeating it in the
wild.

  Module size
     text	   data	    bss	    dec	    hex	filename
    28297	   1040	    700	  30037	   7555	fs/coda/coda.ko.before
    28263	    980	    700	  29943	   74f7	fs/coda/coda.ko.after

Link: http://lkml.kernel.org/r/a2c27663ec4547018c92d71c63b1dff4650b6546.1558117389.git.jaharkes@cs.cmu.edu
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Mikko Rapeli <mikko.rapeli@iki.fi>
Cc: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Yann Droneaud <ydroneaud@opteya.com>
Cc: Zhouyang Jia <jiazhouyang09@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/coda/cnode.c b/fs/coda/cnode.c
index e2dcf2ad..06855f6 100644
--- a/fs/coda/cnode.c
+++ b/fs/coda/cnode.c
@@ -148,6 +148,16 @@ struct inode *coda_fid_to_inode(struct CodaFid *fid, struct super_block *sb)
 	return inode;
 }
 
+struct coda_file_info *coda_ftoc(struct file *file)
+{
+	struct coda_file_info *cfi = file->private_data;
+
+	BUG_ON(!cfi || cfi->cfi_magic != CODA_MAGIC);
+
+	return cfi;
+
+}
+
 /* the CONTROL inode is made without asking attributes from Venus */
 struct inode *coda_cnode_makectl(struct super_block *sb)
 {