s390/ptrace: Convert function macros to inline functions

Convert the function macros user_mode(), instruction_pointer(), and
user_stack_pointer() to inline functions, to align their definition
with x86 and arm64.

Use const qualifier on struct pt_regs pointer parameters to prevent
compiler warnings:

arch/s390/kernel/stacktrace.c: In function ‘arch_stack_walk_user_common’:
arch/s390/kernel/stacktrace.c:114:34: warning: passing argument 1 of
‘instruction_pointer’ discards ‘const’ qualifier from pointer target
type [-Wdiscarded-qualifiers]
...
arch/s390/kernel/stacktrace.c:117:48: warning: passing argument 1 of
‘user_stack_pointer’ discards ‘const’ qualifier from pointer target
type [-Wdiscarded-qualifiers]
...

While at it add const qualifier to all struct pt_regs pointer parameters
that are accessed read-only and use __always_inline instead of inline to
harmonize the helper functions.

[hca@linux.ibm.com: Use psw_bits() helper]

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>