net/smc: Add size match for smc_buf_get_slot

sangfor inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I76JHC



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This add the ability to get a unused smc_buf_desc based on the buf size
which ensures that the size of obtained smc_buf_desc is the same as the
size set.

Signed-off-by: Litao Jiao <jiaolitao@sangfor.com.cn>