xfs: Use xchg() in xlog_cil_insert_pcp_aggregate()

try_cmpxchg() loop with constant "new" value can be substituted
with just xchg() to atomically get and clear the location.

The code on x86_64 improves from:

    1e7f:	48 89 4c 24 10       	mov    %rcx,0x10(%rsp)
    1e84:	48 03 14 c5 00 00 00 	add    0x0(,%rax,8),%rdx
    1e8b:	00
			1e88: R_X86_64_32S	__per_cpu_offset
    1e8c:	8b 02                	mov    (%rdx),%eax
    1e8e:	41 89 c5             	mov    %eax,%r13d
    1e91:	31 c9                	xor    %ecx,%ecx
    1e93:	f0 0f b1 0a          	lock cmpxchg %ecx,(%rdx)
    1e97:	75 f5                	jne    1e8e <xlog_cil_commit+0x84e>
    1e99:	48 8b 4c 24 10       	mov    0x10(%rsp),%rcx
    1e9e:	45 01 e9             	add    %r13d,%r9d

to just:

    1e7f:	48 03 14 cd 00 00 00 	add    0x0(,%rcx,8),%rdx
    1e86:	00
			1e83: R_X86_64_32S	__per_cpu_offset
    1e87:	31 c9                	xor    %ecx,%ecx
    1e89:	87 0a                	xchg   %ecx,(%rdx)
    1e8b:	41 01 cb             	add    %ecx,%r11d

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Chandan Babu R <chandan.babu@oracle.com>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Alex Elder <elder@riscstar.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>