seqlock: annotate spinning as unlikely() in __read_seqcount_begin

Annotation already used to be there, but got lost in 52ac39e
("seqlock: seqcount_t: Implement all read APIs as statement expressions").
Does not look like it was intentional.

Without it gcc 12 decides to compile the following in path_init:
        nd->m_seq = __read_seqcount_begin(&mount_lock.seqcount);
        nd->r_seq = __read_seqcount_begin(&rename_lock.seqcount);

into 2 cases of conditional jumps forward if the value is even, aka
branch prediction miss by default in the common case on x86-64.

With the patch jumps are only for odd values.

before:
[snip]
    mov    0x104fe96(%rip),%eax        # 0xffffffff82409680 <mount_lock>
    test   $0x1,%al
    je     0xffffffff813b97fa <path_init+122>
    pause
    mov    0x104fe8a(%rip),%eax        # 0xffffffff82409680 <mount_lock>
    test   $0x1,%al
    jne    0xffffffff813b97ee <path_init+110>
    mov    %eax,0x48(%rbx)
    mov    0x104fdfd(%rip),%eax        # 0xffffffff82409600 <rename_lock>
    test   $0x1,%al
    je     0xffffffff813b9813 <path_init+147>
    pause
    mov    0x104fdf1(%rip),%eax        # 0xffffffff82409600 <rename_lock>
    test   $0x1,%al
    jne    0xffffffff813b9807 <path_init+135>
[/snip]

after:
[snip]
   mov    0x104fec6(%rip),%eax        # 0xffffffff82409680 <mount_lock>
   test   $0x1,%al
   jne    0xffffffff813b99af <path_init+607>
   mov    %eax,0x48(%rbx)
   mov    0x104fe35(%rip),%eax        # 0xffffffff82409600 <rename_lock>
   test   $0x1,%al
   jne    0xffffffff813b999d <path_init+589>
[/snip]

Interestingly .text gets slightly smaller (as reported by size(1)):
before:	20702563
after:	20702429

Signed-off-by: Mateusz Guzik <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: mjguzik

include/linux/seqlock.h

@@ -272,7 +272,7 @@ SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     mutex)
 ({									\
 	unsigned __seq;							\
 									\
-	while ((__seq = seqprop_sequence(s)) & 1)			\
+	while (unlikely((__seq = seqprop_sequence(s)) & 1))		\
 		cpu_relax();						\
 									\
 	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);			\