cxgbei: Rework the pdu_append_data hook to support M_WAITOK.

• Only allocate 16K jumbo mbufs if the region of data to be appended is sufficiently large, and use a loop.

• Use m_getm2() to allocate a chain for data less than 16K, or if m_getjcl() fails.

• Use ENOMEM as the return value instead of '1' if the hook fails due to a memory allocation error.

Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D29909