blk: clear error flag when setting zero flag

Ref: pmem/issues#4
Merge pull request #40 from plebioda/btt-clear-error-flag-when-setting-zero-flag

blk: clear error flag when setting zero flag
common: fix format strings for off_t and uint64_t

This fixes some warnings when using 32-bit compiler.
Merge pull request #41 from krzycz/common-print_formats

common: fix format strings for off_t and uint64_t