fs: inline step_into() and walk_component()

fs: inline step_into() and walk_component()

The primary consumer is link_path_walk(), calling walk_component() every
time which in turn calls step_into().

Inlining these saves overhead of 2 function calls per path component,
along with allowing the compiler to do better job optimizing them in place.

step_into() had absolutely atrocious assembly to facilitate the
slowpath. In order to lessen the burden at the callsite all the hard
work is moved into step_into_slowpath() and instead an inline-able
fastpath is implemented for rcu-walk.

The new fastpath is a stripped down step_into() RCU handling with a
d_managed() check from handle_mounts().

Benchmarked as follows on Sapphire Rapids:
1. the "before" was a kernel with not-yet-merged optimizations (notably
   elision of calls to security_inode_permission() and marking ext4
   inodes as not having acls as applicable)
2. "after" is the same + the prep patch + this patch
3. benchmark consists of issuing 205 calls to access(2) in a loop with
   pathnames lifted out of gcc and the linker building real code, most
   of which have several path components and 118 of which fail with
   -ENOENT.

Result in terms of ops/s:
before:	21619
after:	22536 (+4%)

profile before:
  20.25%  [kernel]                  [k] __d_lookup_rcu
  10.54%  [kernel]                  [k] link_path_walk
  10.22%  [kernel]                  [k] entry_SYSCALL_64
   6.50%  libc.so.6                 [.] __GI___access
   6.35%  [kernel]                  [k] strncpy_from_user
   4.87%  [kernel]                  [k] step_into
   3.68%  [kernel]                  [k] kmem_cache_alloc_noprof
   2.88%  [kernel]                  [k] walk_component
   2.86%  [kernel]                  [k] kmem_cache_free
   2.14%  [kernel]                  [k] set_root
   2.08%  [kernel]                  [k] lookup_fast

after:
  23.38%  [kernel]                  [k] __d_lookup_rcu
  11.27%  [kernel]                  [k] entry_SYSCALL_64
  10.89%  [kernel]                  [k] link_path_walk
   7.00%  libc.so.6                 [.] __GI___access
   6.88%  [kernel]                  [k] strncpy_from_user
   3.50%  [kernel]                  [k] kmem_cache_alloc_noprof
   2.01%  [kernel]                  [k] kmem_cache_free
   2.00%  [kernel]                  [k] set_root
   1.99%  [kernel]                  [k] lookup_fast
   1.81%  [kernel]                  [k] do_syscall_64
   1.69%  [kernel]                  [k] entry_SYSCALL_64_safe_stack

While walk_component() and step_into() of course disappear from the
profile, the link_path_walk() barely gets more overhead despite the
inlining thanks to the fast path added and while completing more walks
per second.

I did not investigate why overhead grew a lot on __d_lookup_rcu().

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251120003803.2979978-2-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>