codecov: keep "inline" for ALWAYS_INLINE

Previous commit c31e6591 changed
the ALWAYS_INLINE macros to avoid functions being inlined for
the purpose of code coverage. This has a side effect of causing
the text sections of kobject_hash.c and priv_stacks_hash.c
to ballon more than 10 times in size. This is caused by
attaching the unused attribute, which results in all those
functions being in the text sections though they are never
used. So just keep the "inline" there.

This also removes -fno-inline from NO_COVERAGE_FLAGS so these
two files are not compiled with flags related to code coverage.

Fixes #15009

Signed-off-by: Daniel Leung <daniel.leung@intel.com>