btrfs: remove redundant check in up check_setget_bounds

There are two separate checks in the bounds checker, the first one being
a special case of the second. As this function is performance critical
due to checking access to any eb member, reducing the size can slightly
improve performance.

On a release build on x86_64 the helper is completely inlined so the
function call overhead is also gone.

There was a report of 5% performance drop on metadata heavy workload,
that disappeared after disabling asserts. The most significant part of
that is the bounds checker.

https://lore.kernel.org/linux-btrfs/20200724164147.39925-1-josef@toxicpanda.com/

After the analysis, the optimized code removes the worst overhead which
is the function call and the performance was restored.

https://lore.kernel.org/linux-btrfs/20200730110943.GE3703@twin.jikos.cz/



1. baseline, asserts on, setget check on

run time:		46s
run time with perf:	48s

2. asserts on, comment out setget check

run time:		44s
run time with perf:	47s

So this is confirms the 5% difference

3. asserts on, optimized seget check

run time:		44s
run time with perf:	47s

The optimizations are reducing the number of ifs to 1 and inlining the
hot path. Low-level stuff, gets the performance back. Patch below.

4. asserts off, no setget check

run time:		44s
run time with perf:	45s

This verifies that asserts other than the setget check have negligible
impact on performance and it's not harmful to keep them on.

Analysis where the performance is lost:

* check_setget_bounds is short function, but it's still a function call,
  changing the flow of instructions and given how many times it's
  called the overhead adds up

* there are two conditions, one to check if the range is
  completely outside (member_offset > eb->len) or partially inside
  (member_offset + size > eb->len)

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>