fs/squashfs: fix reading of fragmented files

fs/squashfs: fix reading of fragmented files

The fragmented files were not correctly read because of two issues:

- The squashfs_file_info struct has a field named 'comp', which tells if
the file's fragment is compressed or not. This field was always set to
'true' in sqfs_get_regfile_info and sqfs_get_lregfile_info. It should
actually take sqfs_frag_lookup's return value. This patch addresses
these two assignments.

- In sqfs_read, the fragments (compressed or not) were copied to the
output buffer through a for loop which was reading data at the wrong
offset. Replace these loops by equivalent calls to memcpy, with the
right parameters.

I tested this patch by comparing the MD5 checksum of a few fragmented
files with the respective md5sum output in sandbox, considering both
compressed and uncompressed fragments.

Signed-off-by: Joao Marcos Costa <jmcosta944@gmail.com>
Tested-by: Richard Genoud <richard.genoud@posteo.net>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>