ext4: Include forgotten start block on fallocate insert range

While doing 'insert range' start block should be also shifted right.
The bug can be easily reproduced by the following test:

    ptr = malloc(4096);
    assert(ptr);

    fd = open("./ext4.file", O_CREAT | O_TRUNC | O_RDWR, 0600);
    assert(fd >= 0);

    rc = fallocate(fd, 0, 0, 8192);
    assert(rc == 0);
    for (i = 0; i < 2048; i++)
            *((unsigned short *)ptr + i) = 0xbeef;
    rc = pwrite(fd, ptr, 4096, 0);
    assert(rc == 4096);
    rc = pwrite(fd, ptr, 4096, 4096);
    assert(rc == 4096);

    for (block = 2; block < 1000; block++) {
            rc = fallocate(fd, FALLOC_FL_INSERT_RANGE, 4096, 4096);
            assert(rc == 0);

            for (i = 0; i < 2048; i++)
                    *((unsigned short *)ptr + i) = block;

            rc = pwrite(fd, ptr, 4096, 4096);
            assert(rc == 4096);
    }

Because start block is not included in the range the hole appears at
the wrong offset (just after the desired offset) and the following
pwrite() overwrites already existent block, keeping hole untouched.

Simple way to verify wrong behaviour is to check zeroed blocks after
the test:

   $ hexdump ./ext4.file | grep '0000 0000'

The root cause of the bug is a wrong range (start, stop], where start
should be inclusive, i.e. [start, stop].

This patch fixes the problem by including start into the range.  But
not to break left shift (range collapse) stop points to the beginning
of the a block, not to the end.

The other not obvious change is an iterator check on validness in a
main loop.  Because iterator is unsigned the following corner case
should be considered with care: insert a block at 0 offset, when stop
variables overflows and never becomes less than start, which is 0.
To handle this special case iterator is set to NULL to indicate that
end of the loop is reached.

Fixes: 331573fe
Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Namjae Jeon <namjae.jeon@samsung.com>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: stable@vger.kernel.org