ext4fs: Fix: Read outside partition error

ext4fs: Fix: Read outside partition error

Use lbaint_t for blknr to avoid overflow in ext4fs_read_file().

Background:

blknr (block number) used in ext4fs_read_file() could be increased to a
very large value and causes a wrap around at 32 bit signed integer max,
thus becomes negative. This results in an out-of-normal range for sector
number (during the assignment delayed_start = blknr) where delayed_start
sector is typed uint64 lbaint_t. This causes the "Read outside partition"
error.

This patch was tested on the Synology DS116 (Armada 385) board, and a
4TB Seagate HDD.

Signed-off-by: Tony Dinh <mibodhi@gmail.com>