linux/fs: simplify interop and correct definition of loff_t

• offsets can be negative, loff_t needs to be signed, it also simplifies interop with the rest of the code base to use off_t than the actual linux definition "long long"
• don't rely on the defining "file" to "linux_file" in interface definitions as that causes heartache with includes

Reviewed by: hps@
MFC after: 1 week
Sponsored by: iX Systems
Differential Revision: https://reviews.freebsd.org/D19274