enic: add admin RQ buffer management

The admin receive queue needs pre-posted DMA buffers for incoming
mailbox messages from VFs. Each buffer is a kzalloc'd region mapped
for DMA (2048 bytes, sufficient for any MBOX message).  Zeroing on
allocation ensures that if a completion reports more bytes than
hardware actually DMA-wrote, the parser reads zero padding rather
than uninitialised heap contents.

Add enic_admin_rq_fill(gfp) to post buffers at open time, and
enic_admin_rq_drain() to unmap and free them at close time.
Wire both into the admin channel open/close paths. The gfp_t
parameter lets the caller pass the allocation context; both current
callers -- channel open and the CQ-poll work handler that refills
after draining (added in the next patch) -- run in process context
and use GFP_KERNEL.

Signed-off-by: Satish Kharat <satishkh@cisco.com>
Link: https://patch.msgid.link/20260812-enic-sriov-v2-admin-channel-v2-v13-3-b3809e448aba@cisco.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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