linuxkpi: Move shmem related functions in it's own file

For drmkpi (D23085) we don't want the Linux struct file as we don't emulate
everything. Also the prototypes should be in shmem_fs.h to have 100%
compatibility with Linux.

Reviewed by: hselasky
MFC after: Maybe
Differential Revision: https://reviews.freebsd.org/D23764