IB/srp: Rename FMR-related variables

The next patch will cause the renamed variables to be shared between
the code for FMR and for FR memory registration. Make the names of
these variables independent of the memory registration mode. This
patch does not change any functionality. The start of this patch was
the changes applied via the following shell command:

sed -i.orig 's/SRP_FMR_SIZE/SRP_MAX_PAGES_PER_MR/g; \
    s/fmr_page_mask/mr_page_mask/g;s/fmr_page_size/mr_page_size/g; \
    s/fmr_page_shift/mr_page_shift/g;s/fmr_max_size/mr_max_size/g; \
    s/max_pages_per_fmr/max_pages_per_mr/g;s/nfmr/nmdesc/g; \
    s/fmr_len/dma_len/g' drivers/infiniband/ulp/srp/ib_srp.[ch]

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>