ext4: Fix comparision of unsigned expression with < 0

In file ext4fs.c funtion ext4fs_read_file() compares an
unsigned expression with < 0 like below

	lbaint_t blknr;
	blknr = read_allocated_block(&(node->inode), i);
	if (blknr < 0)
		return -1;

blknr is of type ulong/uint64_t. read_allocated_block() returns
long int. So comparing blknr with < 0 will always be false. Instead
declare blknr as long int.

Similarly ext4/dev.c does a similar comparison. Drop the redundant
comparison.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>