fs: fix a few common typos in source code comments

- s/quadradically/quadratically/
- s/persistant/persistent/

Obtained from:	NetBSD
MFC after:	3 days

sys/fs/devfs/devfs_vnops.c

@@ -1345,7 +1345,7 @@ devfs_pathconf(struct vop_pathconf_args *ap)
 #ifdef MAC
 		/*
 		 * If MAC is enabled, devfs automatically supports
-		 * trivial non-persistant label storage.
+		 * trivial non-persistent label storage.
 		 */
 		*ap->a_retval = 1;
 #else

sys/fs/ext2fs/ext2_alloc.c

@@ -90,13 +90,13 @@ static daddr_t  ext2_mapsearch(struct m_ext2fs *, char *, daddr_t);
  *   1) allocate the requested block.
  *   2) allocate a rotationally optimal block in the same cylinder.
  *   3) allocate a block in the same cylinder group.
- *   4) quadradically rehash into other cylinder groups, until an
+ *   4) quadratically rehash into other cylinder groups, until an
  *        available block is located.
  * If no block preference is given the following hierarchy is used
  * to allocate a block:
  *   1) allocate a block in the cylinder group that contains the
  *        inode for the file.
- *   2) quadradically rehash into other cylinder groups, until an
+ *   2) quadratically rehash into other cylinder groups, until an
  *        available block is located.
  */
 int
@@ -744,7 +744,7 @@ ext2_blkpref(struct inode *ip, e2fs_lbn_t lbn, int indx, e2fs_daddr_t *bap,
  *
  * The policy implemented by this algorithm is:
  *   1) allocate the block in its requested cylinder group.
- *   2) quadradically rehash on the cylinder group number.
+ *   2) quadratically rehash on the cylinder group number.
  *   3) brute force search for a free block.
  */
 static e4fs_daddr_t