common/bl: document and slightly optimize ptr::release().

common/bl: document and slightly optimize ptr::release().

Before the patch GCC was generating:
```
  10f9910:   mov    (%rdi),%rax
  10f9913:   test   %rax,%rax
  10f9916:   je     10f9951 <ceph::buffer::v15_2_0::ptr::release()+0x41>
  10f9918:   mov    0x2c(%rax),%eax
  10f991b:   cmp    $0x1,%eax
  10f991e:   jne    10f9940 <ceph::buffer::v15_2_0::ptr::release()+0x30>
  10f9920:   mov    (%rdi),%rax
  10f9923:   movq   $0x0,(%rdi)
  10f992a:   test   %rax,%rax
  10f992d:   je     10f9958 <ceph::buffer::v15_2_0::ptr::release()+0x48>
  10f992f:   mov    (%rax),%rdx
  10f9932:   mov    %rax,%rdi
  10f9935:   mov    0x8(%rdx),%rdx
  10f9939:   jmpq   *%rdx
  10f993b:   nopl   0x0(%rax,%rax,1)
  10f9940:   mov    (%rdi),%rax
  10f9943:   lock subl $0x1,0x2c(%rax)
  10f9948:   je     10f9920 <ceph::buffer::v15_2_0::ptr::release()+0x10>
  10f994a:   movq   $0x0,(%rdi)
  10f9951:   retq
  10f9952:   nopw   0x0(%rax,%rax,1)
  10f9958:   retq
  10f9959:   nop
  10f995a:   nopw   0x0(%rax,%rax,1)
```

after:
```
  11209b0:   mov    (%rdi),%rax
  11209b3:   movq   $0x0,(%rdi)
  11209ba:   test   %rax,%rax
  11209bd:   je     11209e6 <ceph::buffer::v15_2_0::ptr::release()+0x36>
  11209bf:   mov    0x2c(%rax),%edx
  11209c2:   lea    0x2c(%rax),%rcx
  11209c6:   cmp    $0x1,%edx
  11209c9:   jne    11209e0 <ceph::buffer::v15_2_0::ptr::release()+0x30>
  11209cb:   mov    (%rax),%rdx
  11209ce:   mov    %rax,%rdi
  11209d1:   mov    0x8(%rdx),%rdx
  11209d5:   jmpq   *%rdx
  11209d7:   nopw   0x0(%rax,%rax,1)
  11209de:
  11209e0:   lock subl $0x1,(%rcx)
  11209e4:   je     11209cb <ceph::buffer::v15_2_0::ptr::release()+0x1b>
  11209e6:   retq
```

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