tests/kernel/mmap-bmap: Add bmap mmap test

eCryptfs bmap is not yet being exercised by the eCryptfs tests so add
a simple FIBMAP test that checks to see if an encrypted file blocks are
contained in the lower file system.  For lower file systems that don't
support FIBMAP this test is effectively always a no-op.  Checked against
kernel gcov to see if bmap is being exercised.
tests/kernel/mknod.sh: Add simple mknod test

eCryptfs mknod is not yet being exercised by the eCryptfs tests so add
a simple test that creates the 'full' char device (1:7) and checks if
it looks sane. Checked against kernel gcov to see if ecryptfs_mknod
is being exercised.
tests/kernel/link.sh: Add simple hard link test
  
  eCryptfs link is not yet being exercised by the eCryptfs tests so add
  a simple test that creates a hard link to a text file and run some simple
  sanity checks on the link.  Checked against kernel gcov to see if
  ecryptfs_link is being exercised.
tests/kernel/xattr.sh: Add simple xattr test

eCryptfs xattr is not yet being exercised by the eCryptfs tests so add
a simple test that sets a few xattrs, lists them and reads the values back.
This exercises ecryptfs_listxattr and ecryptfs_removexattr. Checked using
kernel gcov.
* === added directory tests/kernel/mmap-bmap, === added directory
  tests/kernel/xattr, tests/kernel/link.sh, tests/kernel/Makefile.am,
  tests/kernel/mknod.sh, tests/kernel/mmap-bmap.sh, tests/kernel/mmap-
  bmap/test.c, tests/kernel/tests.rc, tests/kernel/xattr.sh,
  tests/kernel/xattr/test.c:
  - ran the current eCryptfs tests on 3.8-rc4 with kernel gcov enabled
    and spotted a few trivial areas where it would be useful to up the
    test coverage on the code
  - so here are a few very simple additional tests to exercise eCryptfs
    a little further