MFC r294520:

MFC r294520:
LinuxKPI atomic fixes:
- Fix implementation of atomic_add_unless(). The atomic_cmpset_int()
  function returns a boolean and not the previous value of the atomic
  variable.
- The atomic counters should be signed according to Linux.
- Some minor cosmetics and styling while at it.

Reviewed by:	alfred @
Sponsored by:	Mellanox Technologies

git-svn-id: svn://svn.freebsd.org/base/stable/10@297658 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f