locks: move i_lock acquisition into generic_*_lease handlers

Now that we have a saner internal API for managing leases, we no longer
need to mandate that the inode->i_lock be held over most of the lease
code. Push it down into generic_add_lease and generic_delete_lease.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

Documentation/filesystems/vfs.txt

@@ -895,8 +895,9 @@ otherwise noted.
   splice_read: called by the VFS to splice data from file to a pipe. This
 	       method is used by the splice(2) system call
 
-  setlease: called by the VFS to set or release a file lock lease.
-	    setlease has the file_lock_lock held and must not sleep.
+  setlease: called by the VFS to set or release a file lock lease. setlease
+	    implementations should call generic_setlease to record or remove
+	    the lease in the inode after setting it.
 
   fallocate: called by the VFS to preallocate blocks or punch a hole.