kgdb,x86: fix regression in detach handling

The fix from ba773f7c
(x86,kgdb: Fix hw breakpoint regression) was not entirely complete.

The kgdb_remove_all_hw_break() function also needs to call the
hw_break_release_slot() or else a breakpoint can get activated again
after the debugger has detached.

The kgdb test suite exposes the behavior in the form of either a hang
or repetitive failure.  The kernel config that exposes the problem
contains all of the following:

CONFIG_DEBUG_RODATA=y
CONFIG_KGDB_TESTS=y
CONFIG_KGDB_TESTS_ON_BOOT=y
CONFIG_KGDB_TESTS_BOOT_STRING="V1F100"

Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Tested-by: Frederic Weisbecker <fweisbec@gmail.com>