posix: remove R_OK from InodeMustBeSearchableDir()

Checking for read permissions (via R_OK) in InodeMustBeSearchableDir()
is either redundant or incorrect.  Remove it.

Assume dirfd is a file descriptor for a directory whose permission bits
are 0111 (i.e., search permissions only).  Consider the following API
calls:

1. red_openat(dirfd, "..", RED_O_RDONLY)
2. red_openat(dirfd, "./..", RED_O_RDONLY)
3. red_openat(dirfd, ".", RED_O_RDONLY)

Both #1 and #2 should succeed (assuming dirfd's parent directory has
read permissions).  However, InodeMustBeSearchableDir() would cause
both #1 and #2 to fail, since it will check whether dirfd has read
permissions, even though those aren't required.

Case #3 should fail, and does fail, but InodeMustBeSearchableDir() does
not need to be the place where this failure originates, because the
check for read permissions will occur later, in FildesOpen().  The R_OK
check in InodeMustBeSearchableDir() is redundant.

See also commit 80c2c79, the origin of the relevant code.

posix/path.c

@@ -1299,7 +1299,7 @@ static bool IsDotOrDotDot(
 
     @retval 0               Operation was successful: @p ulInode is a directory.
     @retval -RED_EACCES     #REDCONF_POSIX_OWNER_PERM is true and @p ulInode is
-                            a directory without read or search permissions.
+                            a directory without search permissions.
     @retval -RED_EBADF      @p ulInode is not a valid inode.
     @retval -RED_EIO        A disk I/O error occurred.
     @retval -RED_ENOTDIR    @p ulInode is not a directory.
@@ -1335,7 +1335,7 @@ static REDSTATUS InodeMustBeSearchableDir(
       #if REDCONF_POSIX_OWNER_PERM == 1
         if(ret == 0)
         {
-            ret = RedPermCheck(RED_X_OK | RED_R_OK, sb.st_mode, sb.st_uid, sb.st_gid);
+            ret = RedPermCheck(RED_X_OK, sb.st_mode, sb.st_uid, sb.st_gid);
         }
       #endif
     }