device_cgroup: avoid access to ->i_rdev in the common case in devcgroup_inode_permission()

The routine gets called for every path component during lookup.
->i_mode is going to be cached on account of permission checks, while
->i_rdev is an area which is most likely cache-cold.

gcc 14.2 is kind enough to emit one branch:
movzwl (%rbx),%eax
mov %eax,%edx
and $0xb000,%dx
cmp $0x2000,%dx
je 11bc <inode_permission+0xec>

This patch is lazy in that I don't know if the ->i_rdev branch makes
any sense with the newly added mode check upfront. I am not changing any
semantics here though.

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

include/linux/device_cgroup.h

···
 {
 	short type, access = 0;
 
+	if (likely(!S_ISBLK(inode->i_mode) && !S_ISCHR(inode->i_mode)))
+		return 0;
+
 	if (likely(!inode->i_rdev))
 		return 0;
 
 	if (S_ISBLK(inode->i_mode))
 		type = DEVCG_DEV_BLOCK;
-	else if (S_ISCHR(inode->i_mode))
+	else /* S_ISCHR by the test above */
 		type = DEVCG_DEV_CHAR;
-	else
-		return 0;
 
 	if (mask & MAY_WRITE)
 		access |= DEVCG_ACC_WRITE;