If the starting block number of either the source or destination file
exceeds the EOF, EXT4_IOC_MOVE_EXT should return EINVAL.

Also fixed the helper function mext_check_coverage() so that if the
logical block is beyond EOF, make it return immediately, instead of
looping until the block number wraps all the away around.  This takes
long enough that if there are multiple threads trying to do pound on
an the same inode doing non-sensical things, it can end up triggering
the kernel's soft lockup detector.

Reported-by:  <syzbot+c61979f6f2cba5cb3c06@syzkaller.appspotmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org