fs: ext2: Fix nbytes_to_read calculation in ext2_inode_read()

Fix incorrect nbytes_to_read calculation in ext2_inode_read() function.
Previously nbytes_to_read was decremented by read value which caused
incorrect calculation of bytes to read in subsequent iterations.
Now nbytes_to_read is decremented by to_read value which represents
the actual number of bytes read in current iteration.

This fixes potential data corruption issues when reading files from
ext2 filesystem.

Signed-off-by: Chen Shu <751541594@qq.com>