kmod/core: fix kpatch_put_modules call order

kpatch_put_modules() should be called _after_ removing the ftrace
filters.
kmod/core: fix kpatch_put_modules call in error path

If kpatch_ftrace_add_func fails, num_funcs will be one less than what it
needs to be for kpatch_put_modules to work properly.  Instead give it
the full array size, and it can figure out which modules to put based on
whether func->mod is nonzero.
kmod/core: fix error path for kpatch_verify_symbol_match failure

If kpatch_verify_symbol_match() fails, set the num_funcs variable so
that the ftrace rollback only removes the filter for the affected
functions.
kmod/core: protect find_module with module_mutex
kmod/core: use single quotes

Use single quotes when printing the name of a patch module, rather than
double quotes.  This is more consistent with other printk messages, and
looks better too!
kmod/core: add ftrace helper functions

Move all the ftrace filtering and registering logic into a couple of new
helper functions.  Change kpatch_num_registered to kpatch_num_patched,
which now tracks the number of patched functions rather than the number
of patch modules.

This simplifies the code a bit and will also prevent a future loaded
module scenario where ftrace_ops can be registered with an empty filter,
resulting in _all_ kernel functions getting registered with ftrace.
kmod/core: move kpatch_write_relocations call

Write the relocations _after_ calling kpatch_calculate_old_addr() so
that we'll have a reference to the affected modules.
kmod/core: get rid of kpatch_internal

The kpatch_internal struct is a good idea, in that it documents which
parts of kpatch_module shouldn't be used by the patch module.  But it
creates extra code and will require more extra code if we want to keep a
list of kpmods, which is needed to create a module notifier for module
patching of future loaded modules.

Embedding the private data directly in the public struct allows the code
to be simpler: no extra kmallocs/kfrees, no need to store pointers
between the public and private structs.  I think the simpler code is
worth the tradeoff (exposing implementation detail).  Kernel code
usually doesn't bother with hiding a internal struct data from other
kernel code anyway.  For example, see ftrace_ops or struct kprobe.

The private fields are documented with a "private" comment.
kmod/core: create global kpmod_list

Create a list of registered patch modules, which will be used in the
module notifier to patch future loaded modules.  It will also be used to
fix the kpatch replace race condition where it calls module_put too
early.
kmod/core: fix replace race condition

In the replace case, stop calling module_put on a patch module before
we're potentially done with it.

This will also be needed for future module patching if we want to
properly replace a patch module which only patches a future loaded
module (that's a mouthful).

Fixes #165.
kmod/core: support for patching of future loaded modules

This allows a patch module to contain patched functions for modules
which haven't been loaded yet.  If/when the module is loaded later, it
will be patched from the module notifier function.
kmod/core: make dynrelas section writable

On RHEL 7, the core module's write to dynrela->src causes a panic
because the dynrela section is read-only.
Merge pull request #242 from jpoimboe/future-loaded-modules

support for patching future loaded modules