Enhancement when writing an entire block of a file.

Merge from UFS r231313:

This change first attempts the uiomove() to the newly allocated
(and dirty) buffer and only zeros it if the uiomove() fails. The
effect is to eliminate the gratuitous zeroing of the buffer in
the usual case where the uiomove() successfully fills it.

MFC after:	3 days
Document the "gssname" and "allgssname" mount options added by the
host-based initiator credential patches.
This is a content change.
Remove extraneous format string converison specifier

Submitted by:	wxs@