mntent: fix potential mishandling of extremely long lines

mntent: fix potential mishandling of extremely long lines

commit 05973dc3bbc1aca9b3c8347de6879ed72147ab3b made it so that lines
longer than INT_MAX can in theory be read, but did not use a suitable
type for the positions determined by sscanf. we could change to using
size_t, but since the signature for getmntent_r does not admit lines
longer than INT_MAX, it does not make sense to support them in the
legacy thread-unsafe form either -- the principle here is that there
should not be an incentive to use the unsafe function to get added
functionality.