Defect in XSH posix_fadvise

[Geoff Clare <yyy@xxxxxxxxxxxxx>]

@ page 861 line 27996 section posix_fadvise objection [gwc 
fadvise+fallocate_neg_len]

Problem:

Defect code :  1. Error

In TC2 the "len" argument for posix_fadvise() and posix_fallocate() was
changed from type size_t to off_t.  This means that the value of len
can now be negative, but the descriptions have not been updated to
account for this.

There are two choices.  Either make negative len values invalid, or
make them valid and specify how they are handled.  Given that negative
values were previously not possible, and considering the current
treatment of len=0 by the two functions and the statements about
offset+len for posix_fallocate(), I think it would be best for
negative len values to be treated as invalid.  The alternative would
be to make offset and len be treated similarly to l_start and l_len in
struct flock.

Action:

Change the EINVAL entry in the posix_fadvise() ERRORS section from:

    "[EINVAL]   The value of advice is invalid."

to:
    "[EINVAL]   The value of advice is invalid, or the value of len is
                less than zero."

Similarly for
page 863 line 28039 section posix_fallocate objection

Change the EINVAL entry in the posix_fallocate() ERRORS section from:

    "[EINVAL]   The len argument was zero or the offset argument was
                less than zero."

to:
    "[EINVAL]   The len argument is less than or equal to zero, or the
                offset argument is less than zero."