basic/stat-util: make mtime check stricter and use entire timestamp

basic/stat-util: make mtime check stricter and use entire timestamp

Note that st_mtime member of struct stat is defined as follows,

 #define st_mtime st_mtim.tv_sec

Hence we omitted checking nanosecond part of the timestamp (struct
timespec) and possibly would miss modifications that happened within the
same second.

(cherry picked from commit a59b0a9f768f6e27b25f4f1bab6de08842e78d74)

Related: #1642728