debugfs: fix string_to_time for MacOS strptime()

debugfs: fix string_to_time for MacOS strptime()

The strptime() function does not update fields in struct tm that are
not specified in the input format.  The glibc implementation sets the
tm_yday field (%j) when any of the year (%Y), month (%m), or day (%d)
fields are changed, but the MacOS strptime() does not set tm_yday in
this case.  This caused string_to_time() to calculate the wrong Unix
epoch on MacOS. If tm_yday is unset, compute it in string_to_time().

This also fixes test regression failures for FreeBSD.

Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>