netfs: Split some core bits out into their own file

netfs: Split some core bits out into their own file

Split some core bits out into their own file.  More bits will be added to
this file later.

Signed-off-by: David Howells <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
cc: [email protected]

Link: https://lore.kernel.org/r/164623006934.3564931.17932680017894039748.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/164678218407.1200972.1731208226140990280.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/164692920944.2099075.11990502173226013856.stgit@warthog.procyon.org.uk/