simplify TRIM_OFF_LEN() in "ltp/fsx.c"

simplify TRIM_OFF_LEN() in "ltp/fsx.c"

A recent commit added a TRIM_OFF_LEN() macro in "ltp/fsx.c":
    5843147e xfstests: fsx fallocate support is b0rked
A later commit fixed a problem with that macro:
    c47d7a51 xfstests: fix modulo-by-zero error in fsx

There is an extra flag parameter in that macro that I didn't like
in either version.  When looking at it the second time around I
concluded that there was no need for the flag after all.

Going back to the first commit, the code that TRIM_OFF_LEN()
replaced had one of two forms:
  - For OP_READ and OP_MAP_READ:
	if (file_size)
		offset %= file_size;
	else
		offset = 0;
	if (offset + size > file_size)
		size = file_size - offset;

  - For all other cases (except OP_TRUNCATE):
	offset %= maxfilelen;
	if (offset + size > maxfilelen)
		size = maxfilelen - offset;

There's no harm in ensuring maxfilelen is non-zero (and doing so
is safer than what's done above).  So both of the above can be
generalized this way:
	if (SIZE_LIMIT)
		offset %= SIZE_LIMIT;
	else
		offset = 0;
	if (offset + size > SIZE_LIMIT)
		size = SIZE_LIMIT - offset;

In other words, there is no need for the extra flag in the macro.

The following patch just does away with it.  It uses the value of
the "size" parameter directly in avoiding a divide-by-zero, and in
the process avoids referencing the global "file_size" within the
macro expansion.

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>