fs/namei.c: Add hashlen_string() function

We'd like to make more use of the highly-optimized dcache hash functions
throughout the kernel, rather than have every subsystem create its own,
and a function that hashes basic null-terminated strings is required
for that.

(The name is to emphasize that it returns both hash and length.)

It's actually useful in the dcache itself, specifically d_alloc_name().
Other uses in the next patch.

full_name_hash() is also tweaked to make it more generally useful:
1) Take a "char *" rather than "unsigned char *" argument, to
   be consistent with hash_name().
2) Handle zero-length inputs.  If we want more callers, we don't want
   to make them worry about corner cases.

Signed-off-by: George Spelvin <linux@sciencehorizons.net>

include/linux/stringhash.h

@@ -1,7 +1,8 @@
 #ifndef __LINUX_STRINGHASH_H
 #define __LINUX_STRINGHASH_H
 
-#include <linux/types.h>
+#include <linux/compiler.h>	/* For __pure */
+#include <linux/types.h>	/* For u32, u64 */
 
 /*
  * Routines for hashing strings of bytes to a 32-bit hash value.
@@ -59,7 +60,7 @@ static inline unsigned long end_name_hash(unsigned long hash)
  *
  * If not set, this falls back to a wrapper around the preceding.
  */
-extern unsigned int full_name_hash(const unsigned char *, unsigned int);
+extern unsigned int __pure full_name_hash(const char *, unsigned int);
 
 /*
  * A hash_len is a u64 with the hash of a string in the low
@@ -69,4 +70,7 @@ extern unsigned int full_name_hash(const unsigned char *, unsigned int);
 #define hashlen_len(hashlen)  ((u32)((hashlen) >> 32))
 #define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash))
 
+/* Return the "hash_len" (hash and length) of a null-terminated string */
+extern u64 __pure hashlen_string(const char *name);
+
 #endif	/* __LINUX_STRINGHASH_H */