Fix typos

Correct various typos in the comments and tests.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Closes #10423

include/os/freebsd/spl/sys/callb.h

@@ -116,7 +116,7 @@ typedef struct callb_cpr {
  *
  *	cp    - ptr to the callb_cpr_t structure for this kernel thread
  *
- *	lockp - pointer to mutex protecting the callb_cpr_t stuct
+ *	lockp - pointer to mutex protecting the callb_cpr_t struct
  *
  *	func  - pointer to the callback function for this kernel thread.
  *		It has the prototype boolean_t <func>(void *arg, int code)

include/os/freebsd/spl/sys/vnode_impl.h

@@ -55,7 +55,7 @@
  * - a 32 bit quantity (xva_mapsize) that specifies the size of the
  *   attribute bitmaps in 32 bit words.
  * - A pointer to the returned attribute bitmap (needed because the
- *   previous element, the requested attribute bitmap) is variable lenth.
+ *   previous element, the requested attribute bitmap) is variable length.
  * - The requested attribute bitmap, which is an array of 32 bit words.
  *   Callers use the XVA_SET_REQ() macro to set the bits corresponding to
  *   the attributes that are being requested.
@@ -79,7 +79,7 @@
  * attributes to be requested/returned.  File systems may or may not support
  * optional attributes.  They do so at their own discretion but if they do
  * support optional attributes, they must register the VFSFT_XVATTR feature
- * so that the optional attributes can be set/retrived.
+ * so that the optional attributes can be set/retrieved.
  *
  * The fields of the xvattr structure are:
  *