ramfs: fix use-after-free when ranaming files

This happens when you move a file to a different directory.

Thread 2 (Thread 2):
 1  0x000000000032ed3b in halt_no_interrupts ()
   at ../../arch/x64/processor.hh:239
   osv::halt () at ../../core/power.cc:28
 2  0x000000000020e7fe in abort (
    msg=msg@entry=0x580160 "page fault outside application")
    at ../../runtime.cc:109
 3  0x00000000002ecfdc in page_fault (ef=0xffffc0003e362008)
    at ../../core/mmu.cc:981
 4  <signal handler called>
 5  merge (b=0xffffc0003b4e8000, a=0xfffffffffffffff8)
    at ../../core/mempool.cc:421
 6  memory::free_page_range_locked (range=range@entry=0xffffc0003b4e8000)
    at ../../core/mempool.cc:436
 7  0x00000000003186d7 in memory::free_page_range (
    range=range@entry=0xffffc0003b4e8000) at ../../core/mempool.cc:448
 8  0x0000000000319280 in free_large (obj=0xffffc0003b4e9000)
    at ../../core/mempool.cc:460
 9  std_free (object=0xffffc0003b4e9000) at ../../core/mempool.cc:790
 10 free (obj=0xffffc0003b4e9000) at ../../core/mempool.cc:892
 11 0x0000000000340e52 in ramfs_free_node (np=np@entry=0xffffc0003d250d40)
    at ../../fs/ramfs/ramfs_vnops.c:78
 12 0x0000000000340ed2 in ramfs_remove_node (dnp=0xffffc0003d250a00,
    np=0xffffc0003d250d40) at ../../fs/ramfs/ramfs_vnops.c:131
 13 0x0000000000340ffd in ramfs_rename (dvp1=0xffffc0003fc9a180,
    vp1=0xffffc0003fc9a100, name1=<optimized out>, dvp2=0xffffc0003fc9a280,
    vp2=<optimized out>, name2=0x2000001fedf1 "prefix_suffix")
    at ../../fs/ramfs/ramfs_vnops.c:380
 14 0x0000000000340069 in sys_rename (src=<optimized out>,
    dest=<optimized out>) at ../../fs/vfs/vfs_syscalls.c:671
 15 0x000000000033a3b2 in rename (oldpath=<optimized out>,
    newpath=0xffffc0003e858cc0 "/tmp/t3-26e1b47c/prefix_suffix")
    at ../../fs/vfs/main.cc:634

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
vfs: Fix lookup for paths ending with slash

For example, assuming the following file exists:

  /tmp/dir/x

..the following was sondiered absent:

  /tmp/dir/

..this was considered present:

  /tmp/dir

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>