common: make ceph_clock_now() inlineable.

common: make ceph_clock_now() inlineable.

The `ceph_clock_now()` is a widely spread but thin routine.
All it does is wrap `clock_gettime` or `gettimeofday` with
accompanying conversion to `utime_t`.

Unfortunately, as it is defined outside of header, compilers
are enforced to generate a full-blown function. The overhead
is related not only the well visible stack smashing protection
but also to enforcing callers to go through PLT each time.

Taking into account the time getters are usually *user-space
syscalls* (leveraging e.g. the VDSO mechanism), eradicating
even small boilerplate might be beneficial.

```
0000000000000000 <ceph_clock_now()>:
   0:   48 83 ec 28             sub    $0x28,%rsp
   4:   31 ff                   xor    %edi,%edi
   6:   48 89 e6                mov    %rsp,%rsi
   9:   64 48 8b 04 25 28 00    mov    %fs:0x28,%rax
  10:   00 00
  12:   48 89 44 24 18          mov    %rax,0x18(%rsp)
  17:   31 c0                   xor    %eax,%eax
  19:   e8 00 00 00 00          callq  1e <ceph_clock_now()+0x1e>
  1e:   8b 44 24 08             mov    0x8(%rsp),%eax
  22:   48 c1 e0 20             shl    $0x20,%rax
  26:   48 89 c2                mov    %rax,%rdx
  29:   8b 04 24                mov    (%rsp),%eax
  2c:   48 09 d0                or     %rdx,%rax
  2f:   48 8b 4c 24 18          mov    0x18(%rsp),%rcx
  34:   64 48 33 0c 25 28 00    xor    %fs:0x28,%rcx
  3b:   00 00
  3d:   75 05                   jne    44 <ceph_clock_now()+0x44>
  3f:   48 83 c4 28             add    $0x28,%rsp
  43:   c3                      retq
  44:   e8 00 00 00 00          callq  49 <SubProcess::spawn()::__PRETTY_FUNCTION__+0x9>
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>