fs-verity: remove unnecessary extern keywords

Remove the unnecessary 'extern' keywords from function declarations.
This makes it so that we don't have a mix of both styles, so it won't be
ambiguous what to use in new fs-verity patches.  This also makes the
code shorter and matches the 'checkpatch --strict' expectation.

Link: https://lore.kernel.org/r/20200511192118.71427-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/verity/fsverity_private.h

@@ -134,7 +134,7 @@ void __init fsverity_check_hash_algs(void);
 
 /* init.c */
 
-extern void __printf(3, 4) __cold
+void __printf(3, 4) __cold
 fsverity_msg(const struct inode *inode, const char *level,
 	     const char *fmt, ...);
 

include/linux/fsverity.h

@@ -121,23 +121,23 @@ static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
 
 /* enable.c */
 
-extern int fsverity_ioctl_enable(struct file *filp, const void __user *arg);
+int fsverity_ioctl_enable(struct file *filp, const void __user *arg);
 
 /* measure.c */
 
-extern int fsverity_ioctl_measure(struct file *filp, void __user *arg);
+int fsverity_ioctl_measure(struct file *filp, void __user *arg);
 
 /* open.c */
 
-extern int fsverity_file_open(struct inode *inode, struct file *filp);
-extern int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr);
-extern void fsverity_cleanup_inode(struct inode *inode);
+int fsverity_file_open(struct inode *inode, struct file *filp);
+int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr);
+void fsverity_cleanup_inode(struct inode *inode);
 
 /* verify.c */
 
-extern bool fsverity_verify_page(struct page *page);
-extern void fsverity_verify_bio(struct bio *bio);
-extern void fsverity_enqueue_verify_work(struct work_struct *work);
+bool fsverity_verify_page(struct page *page);
+void fsverity_verify_bio(struct bio *bio);
+void fsverity_enqueue_verify_work(struct work_struct *work);
 
 #else /* !CONFIG_FS_VERITY */