Fix witness warning: uma_zalloc->witness_warn, exclusive sleep mutex xforms_list (IPsec transforms list)

[ae]

Summary

After rS336439 crypto_newsession() does uma_zalloc() with M_WAITOK flag and since, xform_init holds mutex during this call, WITNESS shows warning about possible deadlock.
This patch does several things:

• xform-related functions moved into subr_ipsec.c
• release XFORMS_LOCK() before invoking xf_init() that calls crypto_newsession()
• use ipsec_kmod_enter/exit/drain functions to protect from xform kernel module unloading during xfrom_init call
• always build ipsec_kmod_enter/exit/drain functions into kernel (when options IPSEC or IPSEC_SUPPORT is specified)
• added new field xf_cntr to struct xformsw, it is used by ipsec_kmod_* functions
• constified xf_name field

Test Plan

I tested the GENERIC kernel, GENERC w/o IPSEC, and GENERIC w/o IPSEC_SUPPORT. Seems all works as expected.

[bz]

Moving functions should be a separate commit and not be mixed with the functional changes.

[ae]

In D17302#368848, @bz wrote:

Moving functions should be a separate commit and not be mixed with the functional changes.

Moving is done, because functions are static, and something should be changed - either ipsec_kmod_* functions should be changed to global, or xform_* moved into subr_ipsec.c, or new functions like ipsec_kmod_* should be added to key.c