linux/fs: simplify interop and correct definition of loff_t

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