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