btrfs: introduce btrfs_write_check()

btrfs_write_check() checks write parameters in one place before
beginning a write. This does away with inode_unlock() after every check.
In the later patches, it will help push inode_lock/unlock() in buffered
and direct write functions respectively.

generic_write_checks needs to be called before as it could truncate
iov_iter and its return used as count.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>