xprtrdma: Decouple frwr_wp_create from frwr_map

frwr_wp_create is the only caller of frwr_map outside the encode
path. It registers a single 4-byte write-pad region from a stack-
local rpcrdma_mr_seg. Inlining the registration logic directly
(sg_init_table + sg_set_page + ib_dma_map_sg + ib_map_mr_sg +
IOVA mangle + reg_wr setup) eliminates the coupling that would
otherwise complicate the removal of rpcrdma_mr_seg from frwr_map's
interface.

The inlined version adds a proper error-unwind ladder: on failure,
the DMA mapping (if established) is released, ep->re_write_pad_mr is
cleared, and the MR is returned to the transport free list. The old
frwr_map-based code relied on rpcrdma_mrs_destroy at teardown to
reclaim partially-initialized MRs.

This is a one-time setup path; duplicating ~20 lines is a reasonable
tradeoff for decoupling the write-pad registration from the data-
path MR registration.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

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