fs: fat: handle deleted directory entries correctly

fs: fat: handle deleted directory entries correctly

Unlink test for FAT file system seems to fail at test_unlink2.
(When I added this test, I haven't seen any errors though.)
for example,
===8<===
fs_obj_unlink = ['fat', '/home/akashi/tmp/uboot_sandbox_test/128MB.fat32.img']

    def test_unlink2(self, u_boot_console, fs_obj_unlink):
        """
        Test Case 2 - delete many files
        """
        fs_type,fs_img = fs_obj_unlink
        with u_boot_console.log.section('Test Case 2 - unlink (many)'):
            output = u_boot_console.run_command('host bind 0 %s' % fs_img)

            for i in range(0, 20):
                output = u_boot_console.run_command_list([
                    '%srm host 0:0 dir2/0123456789abcdef%02x' % (fs_type, i),
                    '%sls host 0:0 dir2/0123456789abcdef%02x' % (fs_type, i)])
                assert('' == ''.join(output))

            output = u_boot_console.run_command(
                '%sls host 0:0 dir2' % fs_type)
>           assert('0 file(s), 2 dir(s)' in output)
E           AssertionError: assert '0 file(s), 2 dir(s)' in '            ./\r\r\n            ../\r\r\n        0   0123456789abcdef11\r\r\n\r\r\n1 file(s), 2 dir(s)'

test/py/tests/test_fs/test_unlink.py:52: AssertionError
===>8===

This can happen when fat_itr_next() wrongly detects an already-
deleted directory entry.

File deletion, which was added in the commit f8240ce95d64 ("fs: fat:
support unlink"), is implemented by marking its entry for a short name
with DELETED_FLAG, but related entry slots for a long file name are kept
unmodified. (So entries will never be actually deleted from media.)

To handle this case correctly, an additional check for a directory slot
will be needed in fat_itr_next().

In addition, I added extra comments about long file name and short file
name format in FAT file system. Although they are not directly related
to the issue, I hope it will be helpful for better understandings
in general.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>