iflib: hold the CTX lock in iflib_pseudo_register

iflib: hold the CTX lock in iflib_pseudo_register

From Jake:
The iflib_device_register function takes the CTX lock before calling
IFDI_ATTACH_PRE, and releases it upon finishing the registration.

Mirror this process in iflib_pseudo_register, so that we always hold the
CTX lock during the attach process when registering a pseudo interface
or a regular interface.

This was caught by code inspection while attempting to analyze where the
CTX lock was held.

Submitted by:	Jacob Keller <jacob.e.keller@intel.com>
Reviewed by:	shurd@, erj@
MFC after:	1 week
Sponsored by:	Intel Corporation
Differential Revision:	https://reviews.freebsd.org/D19604