FreeBSD: Clean up the use of ioflags

• Prefer O_* flags over F* flags that mostly mirror O_* flags anyway, but O_* flags seem to be preferred.
• Simplify the code as all the F*SYNC flags were defined as FFSYNC flag.
• Don't define FRSYNC flag, so we don't generate unnecessary ZIL commits.
• Remove EXCL define, FreeBSD ignores the excl argument for zfs_create() anyway.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Closes #13400