sys/vfs/hammer2: Fix -Wpointer-sign warnings on hammer2_dirhash()
Warned on Linux user space.
Change hammer2_dirhash() and user space dirhash() to take
const char* instead of const unsigned char* for entry "name".
* hammer2_dirhash() callers always pass char* or const char*,
except for one in sbin/newfs_hammer2/mkfs_hammer2.c where filename[]
in ondisk inode (unterminated unsigned char array) is used.
* hammer2_dirhash() only compares name with ascii characters,
and passes substring to hammer2_icrc32() which takes const void*,
so it doesn't need to be unsigned.
* Also change hammer2_inode_create_pfs() (another function whose
callers only pass char* or const char* for PFS "name") to take
const char* instead of const uint8_t* so that hammer2_dirhash()
doesn't get warned after changes above.