Use posix_fallocate as default.

Use posix_fallocate as default.

Summary:
Ftruncate does not throw an error on disk-full. This causes Sig-bus in
the case where the database tries to issue a Put call on a full-disk.

Use posix_fallocate for allocation instead of truncate.
Add a check to use MMaped files only on ext4, xfs and tempfs, as
posix_fallocate is very slow on ext3 and older.

Test Plan: make all check

Reviewers: dhruba, chip

Reviewed By: dhruba

CC: adsharma, leveldb
Differential Revision: https://reviews.facebook.net/D9291