zonefs: handle integer overflow in zonefs_fname_to_fno

zonefs: handle integer overflow in zonefs_fname_to_fno

In zonefs the file name in one of the two directories corresponds to the
zone number.

Here Alexey reported a possible integer overflow in zonefs_fname_to_fno(),
where the parsing of the zone number from the file name can overflow the
'long' data type.

Add a check for integer overflows and if the fno 'long' did overflow
return -ENOENT.

Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Fixes: d207794ababe ("zonefs: Dynamically create file inodes when needed")
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>