main: add "-kernel_cache" flag

This option is similar to fuse(8) kernel_cache

Verified using vmtouch.

Without -kernel_cache:

$ dd if=/dev/zero of=foo bs=1M count=10 ; vmtouch -t foo ; vmtouch foo
10+0 records in
10+0 records out
10485760 bytes (10 MB, 10 MiB) copied, 0,0242321 s, 433 MB/s
           Files: 1
     Directories: 0
   Touched Pages: 2560 (10M)
         Elapsed: 0.011159 seconds
           Files: 1
     Directories: 0
  Resident Pages: 0/2560  0/10M  0%
         Elapsed: 0.000993 seconds

With -kernel_cache:

$ dd if=/dev/zero of=foo bs=1M count=10 ; vmtouch -t foo ; vmtouch foo
10+0 records in
10+0 records out
10485760 bytes (10 MB, 10 MiB) copied, 0,0244015 s, 430 MB/s
           Files: 1
     Directories: 0
   Touched Pages: 2560 (10M)
         Elapsed: 0.011564 seconds
           Files: 1
     Directories: 0
  Resident Pages: 2560/2560  10M/10M  100%
         Elapsed: 0.000369 seconds

1 files changed, 1 insertions, 0 deletions

diff --git a/mount.go b/mount.go
index 39f3f3c..9240afc 100644
--- a/mount.go
+++ b/mount.go
@@ -267,6 +267,7 @@ func initFuseFrontend(args *argContainer) (rootNode fs.InodeEmbedder, wipeKeys f
 		ExcludeWildcard: args.excludeWildcard,
 		ExcludeFrom:     args.excludeFrom,
 		Suid:            args.suid,
+		KernelCache:     args.kernel_cache,
 	}
 	// confFile is nil when "-zerokey" or "-masterkey" was used
 	if confFile != nil {