b_kvabase when the buffer is reclaimed. Otherwise, if b_data for the
mapped buffer was adjusted with the page-offset portion of b_offset,
nothing would re-adjust the b_data, which breaks buffer management
code which expects page-aligned b_data (see e.g. bpman_qenter(), which
skips partial pages).
Fix a minor issue with the GB_KVAALLOC requests, which could result in
returning the mapped buffer if the reused buffer is mapped and have
the right amount of KVA reserved.
Improve assertion in the vfs_buf_check_mapped() to catch unmapped
buffers which have their b_data incorrectly adjusted with offset.
Reported and tested by: pho (previous version)
Reviewed by: jeff (previous version)
Sponsored by: The FreeBSD Foundation
6fd04eff66