For posix_fallocate(2) and posix_fadvise(2), return ESPIPE when
underlying file does not have DFLAG_SEEKABLE set [1].

For posix_fallocate(2), simplify error handling logic.  Do return when
fp is not yet referenced.

Noted by:	bde [1]
Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week